Hello,
(Just my personal experience here : Glitches and huge distorsion with RCA outputs on my Bartok ).
I also have a McIntosh pre-amp, a C70.
If not the same, it has a very similar analog part as the C2700 for what concerns the tube section (so not fully balanced either).
I actually had a quite similar issue as you with my Bartok.
Sound through XLR was very thin, with a noticeable lack of bass.
And, although the sound quality was much better with unbalanced RCAs, I ended up having glitches and distorsion coming out of the Bartok at start-up from time to time with the RCAs (could be hit once every four or five full power off/on).
Long story short, my Bartok was faulty.
It went back to dCS facilities where it got fixed earlier this year.
No more issue since it has been returned back to me: the XLR connection sounds now way better than the RCA when connected to the C70, “full of meat” while being quite detailed (I currently use the max 6V output setting on the Bartok, as the C70 can take up to 10V on its XLR entries).
Hope this can help,
Erwan.